1. Field of the Invention
This invention relates to an apparatus provided with a projection optical system, and in particular to a projection optical apparatus in an exposure apparatus for the manufacture of semiconductor elements which controls the projected pattern image of a mask to a desired projected state irrespective of a variation in the optical characteristic of the projection optical system.
2. Description of the Prior Art
Reduction projection type exposure apparatuses have recently become indispensable for the production of super LSI. In the exposure apparatus of this type, use is usually made of a projection lens for reduction-projecting the circuit pattern on a negative (equivalent to a mask) called a reticle onto a semiconductor wafer at a reduction rate of 1/5 or 1/10 and obtaining a resolved image having a line width of 1 .mu.m or more. At present, development of a projection lens which increases its resolving power while keeping a great projection exposure area is advanced to improve the productivity of semiconductor elements. To obtain a higher resolving power, the numerical aperture (N.A.) of the projection lens must be made great, but this necessarily results in a shallow depth of focus.
In order to cope with a variation in the thickness of a wafer on which the formation of a pattern (lithography) is effected and the unevenness of the surface of the wafer, a focus detector utilizing a gap sensor for detecting the spacing between the projection lens and the wafer is incorporated in the exposure apparatus of this type, and on the basis of the detection signal thereof, the automatic focusing for bringing the imaging plane of the projection lens (the image plane of the projected pattern) and the surface of the wafer into coincidence with each other is effected. The depth of focus for obtaining predetermined pattern line width accuracy becomes smaller as the minimum pattern to be transferred is smaller, and where a pattern of line and space of 1 .mu.m is to be transferred, the depth of focus is of the order of .+-.1 .mu.m.
Now, in the projection optical path from the reticle to the wafer, air more or less freely goes into and out of the space between the reticle and the projection lens, the space between a plurality of lens elements in the projection lens and the space between the projection lens and the wafer. The refractive index of air varies dependently of the atmospheric pressure and therefore, the fact that air goes into and out of the projection optical path means that the phenomenon of the imaging plane of the projection lens being fluctuated in the direction of the optic axis by a variation in the atmospheric pressure, i.e., so-called focus fluctuation, is brought about.
Heretofore, it has not been recognized that focus fluctuation is caused by a variation in the atmospheric pressure, but some drift caused solely in the focus detector has been considered to be the cause of focus fluctuation. Therefore, in the production field where the exposure apparatus is handled, at the commencement of daily work, a pattern has been printed on trial on a wafer and the focus detector has been adjusted so that it is judged as the in-focus at such a wafer position that a predetermined resolved image (line width) is obtained. It takes much time and labor to effect the confirmation and adjustment of the focus position by such a method and thus, the throughput of the apparatus is reduced. Moreover, the atmospheric pressure is not constant during the operation of the apparatus, but fluctuates more or less. Therefore, even if the focus detector is adjusted once a day, accurate focusing is effected only by the atmospheric pressure at that point of time. Particularly, when a typehoon or a hurricane whose atmospheric pressure varies locally passes the district in which the production field lies, considerably great focus fluctuation occurs. Moreover, depending on the scale or the course of the typhoon or the hurricane, a variation of the order of 20-30 mmHg in the atmospheric pressure may occur within a very short time (say, about an hour). So, the amounts of fluctuation of the focuses of various projection lenses by a variation in the atmospheric pressure have been actually measured by an experiment or in the field, and as a result, it has been found that in projection lenses of a certain type of structure, the amount of fluctuation of the focus sensitively changes relative to a variation in the atmospheric pressure and that amount itself greatly exceeds the practical depth of focus.